Tubes and like moldings made of a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer [PFA] are resistant to liquid chemicals and hardly contaminate liquids and the like flowing therethrough and, therefore, have so far been used as materials for the piping/tubing system for transferring high-purity liquids, in particular as materials for the piping/tubing system in semiconductor manufacturing equipment, among others. However, PFA tubes have a problem, namely they are fairly high in liquid chemical permeability.
As an outer tube material to be molded in close contact with existing PFA tubes for attaining reductions in liquid chemical permeability, there is described, for example, a polychlorotrifluoroethylene [PCTFE] (cf. e.g. Patent Document 1: Japanese Kokai (Laid-open) Publication H09-137900). However, these tubes still have a problem; namely, when PFA and PCTFE are actually subjected to multilayer extrusion molding, the PCTFE is inferior in thermal stability on the occasion of molding and in stress cracking resistance.
As a CTFE copolymer, there has also been proposed a fluoropolymer having a copolymerization composition comprising 50 to 99.8 mole percent of TFE and/or CTFE, 0.1 to 49.99 mole percent of a fluoromonomer other than TFE and CTFE and 0.01 to 5 mole percent of at least one compound selected from among itaconic acid, citraconic acid and acid anhydrides thereof (cf. e.g. Patent Document 2: Laid-open European Patent Application No. 1375539). This fluoropolymer comprises such a compound as itaconic acid as an essential copolymerization component for attaining improvements in bond strength. However, there is no concrete disclosure at all about a fluoropolymer comprising both TFE and CTFE as copolymerization components.